This invention relates to novel amorphous metal compositions and to the preparation of wires of these and other amorphous metal alloys.
Heretofore, a limited number of amorphous, i.e. noncrystalline or glassy, metal alloys have been prepared. To obtain the amorphous state, a molten alloy of a suitable composition must be quenched rapidly, or alternatively, a deposition technique must be used: suitably employed vapor deposition, sputtering, electro-deposition, or chemical (electro-less) deposition can be used to produce the amorphous metal.
The production of amorphous metal by these known techniques, i.e. either through a rapid quench of the melt or by deposition, severely limits the form in which the amorphous metal can be obtained. For example, when the amorphous metal is obtained from the melt, the rapid quench has generally been achieved by spreading the molten alloy in a thin layer against a metal substrate such as Cu or Al held at or below room temperature. The molten metal is typically spread to a thickness of about 0.002" which, as discussed in detail by R. Predecki, A. W. Mullendore and N. J. Grant in Trans. AIME 233, 1581 (1965) and R. C. Ruhl in Mat. Sci. & Eng. 1, 313 (1967), leads to a cooling rate of about 10.sup.6 .degree. C./sec.
Various procedures have been proposed to provide rapid quenching by spreading the molten liquid in a thin layer against a metal substrate. Typical examples of such techniques are the gun technique of P. Duwez and R. H. Willens described in Trans. AIME 227, 362 (1963) in which a gaseous shock wave propels a drop of molten metal against a substrate made of a metal such as copper; the piston and anvil technique described by P. Pietrokowsky in Rev. Sci. Instr. 34, 445 (1963) in which two metal plates come together rapidly and flatten out and quench a drop of molten metal falling between them; the casting technique described by R. Pond, Jr. and R. Maddin in Trans. Met. Soc. AIME 245, 2475 (1969) in which a molten metal stream impinges on the inner surface on a rapidly rotating hollow cylinder open at one end; and the rotating double rolls technique described by H. S. Chen and C. E. Miller in Rev. Sci. Instrum. 41, 1237 (1970) in which the molten metal is squirted into the nip of a pair of rapidly rotating metal rollers. These techniques produce small foils or ribbon-shaped samples in which one dimension is much smaller than the other two so that their usefulness as a practical matter is severely limited. Because of the high cooling rates necessary to obtain the amorphous state from quenched liquid alloys, it is required that the amorphous metals be formed in a shape which does not preclude adequate quenching, i.e. they must have at least one dimension small enough to permit the sufficiently rapid removal of the heat from the sample.
Metal alloys which are most easily obtained in the amorphous state by rapid quenching or by deposition techniques are mixtures of transition metals with metalloids, i.e. semimetals. In each case, the alloy is approximately 80 atomic percent transition metal and 20 atomic percent metalloid. Examples of alloys of this type reportedly made previously in the amorphous state are Pd.sub.84 Si.sub.16, Pd.sub.79 Si.sub.21, Pd.sub.77.5 Cu.sub.6 Si.sub.16.5, Co.sub.80 P.sub.20, Au.sub.76.9 Ge.sub.13.65 Si.sub.9.45, Ni.sub.81.4 P.sub.18.6, Fe.sub.80 P.sub.13 C.sub.7, Ni.sub.15 Pt.sub.60 P.sub.25, Ni.sub.42.5 Pd.sub.42.5 P.sub.15, Fe.sub.75 P.sub.15 C.sub.10, Mn.sub.75 P.sub.15 C.sub.10, Ni.sub.80 S.sub.20, and Ni.sub.78 B.sub.22 where the subscripts indicate atomic percent.
The cooling rate necessary to achieve the amorphous state, i.e. to avoid crystallization, and the stability of the amorphous state once it is obtained depends upon the composition of the alloy. Some of these alloys are better glass formers than others; these "better" alloys can be obtained in the amorphous state with a lower cooling rate, which in practice may be more readily obtainable, or can be obtained with a greater thickness when quenched from the melt with a given technique.
Generally, there is a small range of compositions surrounding each of the known amorphous compositions where the amorphous state can be obtained. However, apart from quenching the alloys, no practical guideline is known for predicting with certainty which of the multitude of different alloys will yield an amorphous metal with given processing conditions and hence which of the alloys are "better" glass formers.
The amorphous and the crystalline state are distinguished by the respective absence or presence of long range periodicity. Further, the compositional ordering in alloys may be different for the two states. These differences are reflected in th differences in their x-ray diffraction behavior, and accordingly, x-ray diffraction measurements are most often used to distinguish a crystalline from an amorphous substance. Diffractometer traces of an amorphous substance reveal a slowly varying diffracted intensity, in many respects similar to a liquid, while crystalline materials produce a much more rapidly varying diffracted intensity. Also, the physical properties, which depend upon the atomic arrangement, are uniquely different for the crystalline and the amorphous state. The mechanical properties differ substantially for the two states; for example, a 0.002" thick strip of amorphous Pd.sub.80 Si.sub.20 is relatively more ductile and stronger and will deform plastically upon sufficiently severe bending while a similar crystalline strip of the same composition is brittle and weak and will fracture upon identical bending. Further, the magnetic and electrical properties of the two states are different. In each case, the metastable amorphous state will convert to a crystalline form upon heating to a sufficiently high temperature with the evolution of a heat of crystallization.
It should be noted, moreover, that cooling a molten metal to a glass is distinctly different from cooling such a molten metal to the crystalline state. When a liquid is cooled to a glass, the liquid solidifies continuously over a range of tmperature without a discontinuous evolution of a heat of fusion. In contrast, crystallization is a thermodynamic first order transition and thus is associated with a heat of fusion and a specific temperature.